Big trees 3 or more feet in diameter accounted for nearly half the biomass measured at a Yosemite National Park study site, yet represented only 1 percent of the trees growing there.
Andrew Larson, a University of Montana assistant professor of forest ecology, is part of the research team reporting this new finding.
“These trees are the elites of the forest,” Larson said. “Just a few individual trees per acre, especially sugar pines, store a disproportionately large amount of the carbon in this forest.”
Larson is co-author of a paper on the largest quantitative study yet of the importance of big trees in temperate forests. It was published May 2 in PLoS ONE, an online peer-reviewed journal, and can be found at http://dx.plos.org/10.1371/journal.pone.0036131.
Larson’s research partners included James Lutz and James Freund of the University of Washington and Mark Swanson of Washington State. The Smithsonian Center for Tropical Forest Science funded the research.
“As trees become very large, their ecological importance increases dramatically,” Larson said. “They also become increasingly difficult to replace when lost to natural or human-caused mortality.”
Trees in the western U.S. with trunks more than 3 feet across typically are at least 200 years old. Many forests that were heavily harvested in the 19th and 20th centuries, or those that are used as commercial forest lands today, don’t generally have large-diameter trees, snags or large wood on the ground.
In 2009, Larson and other scientists reported that the rate of large-tree mortality had increased during recent decades in old-growth forests across the West. Scientists, including Lutz, also found that density of large-diameter trees declined nearly 25 percent between the 1930s and 1990s in Yosemite National Park, even though the area was never logged. Because of this, scientists have wanted to study a plot large enough to detect forest ecosystem changes involving large trees, including the effects of climate variability and change – possible culprits in the declines.
The new 63-acre study site in the western part of Yosemite National Park is one of the largest, fully mapped plots in the world and the largest old-growth plot in North America. The tally of what’s there, including the counting and tagging of 34,500 live trees, was done by citizen scientists, mainly undergraduate college students, led by Larson, Lutz, Swanson and Freund.
The tally included all above-ground biomass such as live trees, snags, downed woody debris, litter and what’s called duff, the decaying plant matter on the ground under trees. Even when big trees die, they continue to dominate biomass in different ways. For example, 12 percent of standing snags were the remains of large-diameter trees, but they still accounted for 60 percent of the total biomass of snags.
Live and dead biomass totaled 280 tons per acre (652 metric tons per hectare), a figure unmatched by any other forest in the Smithsonian Center for Tropical Forest Science network, a global group of 42 tropical and temperate forest plots that includes the one in Yosemite.
One implication of the research is that land managers may want to pay more attention to existing big trees, the co-authors said. Last year in Yosemite, for example, managers planning to set fires to clear out overgrown brush and densely packed small trees first used data from the study plot to figure out how many large trees to protect. Fire crews then reduced fuels near the base of large trees before igniting the prescribed fire.
“Before the fires were started, crews raked around some of the large trees so debris wouldn’t just sit and burn at the base of the tree and kill the cambium, the tissue under the bark that sustains trees,” Lutz said.
Another finding from the new work is that forest models based either on scaling theory or competition theory, which are useful for younger, more uniform forests, fail to capture how and where large trees occur in forests.
“The findings of Dr. Larson and his colleagues reveal the central role that science and research collaborations will play in guiding society through our common challenges in managing global carbon budgets,” said James Burchfield, dean of the UM forestry college. “These types of discoveries will allow us to make better decisions about how we manage our forests, so future generations might continue to benefit from these extraordinary ecosystems.”
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